Stewing in their own Gravy

Especially applied to a besieged city. The besiegers may leave
the hostile city to suffer from want of food, loss of commerce,
confinement, and so on. The phrase is very old, borrowed perhaps from
the Bible, “Thou shalt not seethe a kid in its mother's milk.” Chaucer
says -

In his own gress I made him frie,
For anger and for verry jalousie.

Prologue to the Wife of Bathes Tale.

We are told that the Russian ambassador, when Louis Philippe
fortified Paris, remarked, if ever again Paris is in insurrection, it “can be made to stew in its own gravy (jus)”; and Bismarck, at the
siege of Paris, in 1871, said, the Germans intend to leave the city “to
seethe in its own milk.” - See Snell: Chronicles of Twyford, p.
295.

“He relieved us out of our purgatory ... after we had been stewing in
our own gravy.” —The London Spy, 1716.